1. The Field of the Invention
The present invention relates generally to ridge waveguide semiconductor lasers, such as those used in optical transceivers. More particularly, the present invention is directed towards improving the yield and reliability of ridge waveguide lasers in an InP/AlInGaAs materials system.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber optics. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Various types of lasers are used to generate the needed electromagnetic radiation to carry data along the optical fibers. For instance, semiconductor lasers, such as InP-based lasers, emitting at wavelengths between about 1.3 microns to 1.55 microns are of interest as laser transmitters for a variety of optical fiber applications, such as use in optical transceivers.
A semiconductor laser includes a waveguide to guide the optical mode. The waveguide is commonly formed by selecting an effective refractive index to be higher in a core region than surrounding regions. In the vertical direction relative to the axis of the as-grown epitaxial layers, the composition of the semiconductor layers can be grown to produce vertical wave guiding. In the lateral direction, there are several different structures that may be used to form a lateral waveguide. One structure that provides lateral waveguiding is a buried heterostructure. In a buried heterostructure laser, a first epitaxial process is used to grow the vertical laser structure. A narrow ridge is then etched down through the active region into the underlying layers. In a subsequent re-growth process, the sides of the ridge are embedded in a semiconductor having an average refractive index less than that of the ridge, thereby forming a lateral waveguide. This results in a substantially planar structure.
An alternative lateral waveguide structure is a ridge waveguide laser. A side view of an exemplary ridge waveguide laser is shown in FIG. 1. In a ridge waveguide laser 10, a single epitaxial growth process is commonly used to grow the vertical laser structure having a bottom cladding layer 14 deposited on a substrate 12, an active layer 16 deposited on bottom cladding layer 14, and a top cladding layer 18 deposited on active layer 16. A narrow ridge 20 is etched down through top cladding layer 18 typically to within several tenths of a micron above active layer 16, such as a multiple quantum well (MQW) active layer. The ridge 20 is commonly coated on its sides by a dielectric insulator 22, with a suitable metal contact layer 24 patterned to form a top contact to ridge 20.
Ridge waveguide lasers have the advantage of requiring fewer epitaxial growth steps compared with a buried heterostructure laser. However, a ridge waveguide laser has the disadvantage that it is more easily damaged during manufacturing than a comparable buried heterostructure laser. Since a manufacturing process for forming edge-emitting lasers requires considerable handling, non-planar nature of a ridge waveguide laser makes it more susceptible to mechanical damage than a comparable buried heterostructure laser. This is especially true since the manufacturing process commonly includes cleaving laser bars, dicing the laser bars into individual laser die, and then packaging individual die.
During the manufacturing process, even a small mechanical force imposed on a portion of a ridge may generate a high local pressure. In some cases, the ridge may be scratched, reducing yield. In other cases, mechanical damage is introduced into the ridge that shortens the lifetime of the lasers. While techniques exist to planarize ridge waveguide lasers with polymers, such as polyimide, these planarization techniques have the disadvantage of significantly increasing the number of fabrication steps. Additionally, this technique reduces yield, increases thermal resistance, and increases mechanical stress to the ridge and hence the ridge guide laser.
What is desired is an improved ridge guide laser structure with improved yield and reliability, while alleviating the problems identified with manufacturing the ridge guide laser.